Ion implantation is a standard technique for introducing conductivity—altering impurities into semiconductor wafers. A desired impurity material may be ionized in an ion source, the ions may be accelerated to form an ion beam of prescribed energy, and the ion beam may be directed at a front surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material. The ion beam may be distributed over the wafer area by beam scanning, by wafer movement, or by a combination of beam scanning and wafer movement.
Differing kinetic energy may be imparted to the ions of the ion beam. The imparted energy, as well as other factors such as the mass of the implanting ions, may affect the implanted depth of the ions into the semiconductor wafer. In general, a lower energy would result in a shallower implant depth and a higher energy would result in a deeper implant depth with all other parameters equal. A higher kinetic energy may be considered energy greater than about 1,000 kiloelectronvolts (keV) or 1 MeV.
An energy range of an ion implanter may be extended by producing and transporting multiply charged ions since the kinetic energy of the ions is equal to the charge state multiplied by a potential difference through which the ions pass. For example, a potential difference of 400 kilovolts (kV) provides up to 400 keV of energy for singly charged ions, 800 keV for doubly charged ions, and 1,200 keV (or 1.2 MeV) for triply charged ions. Therefore, for the same potential difference, a higher energy is achieved with multiply charged ions.
However, a drawback of multiply charged ions is that undesired charge exchanges of ions may result in contamination that is difficult to filter before it reaches the target wafer. For instance, in an ion implanter having a scanner to scan a spot beam and an angle corrector magnet positioned downstream of the scanner to collimate the scanned ion beam, any contamination occurring before the angle corrector magnet may not be able to be sufficiently deflected by the angle corrector magnet away from the target wafer before some contamination ions strike the wafer. In one example, triply charged phosphorous ions (P+++) may be desired for implantation into a target wafer. Contamination upstream of the angle corrector magnet may cause the P+++ ions to gain or lose a positive charge, introducing parasitic beamlets of P++++ and P++ contamination ions respectively. As used herein, a “parasitic beamlet” is a beamlet formed by streams of charge-altered ions having a charge state different than the desired charge state. Compared to the P+++ ions, the P++++ ions will be bent more by the magnetic field of the angle corrector magnet and the P++ ions will be bent less by the magnetic field of the angle corrector magnet. Unfortunately, some of the P++ contamination ions input to the corrector magnet would still strike the target wafer. The P++ contamination ions that strike the wafer would do so at unintended incident angles and can therefore adversely affect the dopant profile in the target wafer. In addition, the P++ contamination ions can also adversely affect the dose uniformity of implanted ions since a typical dosimetry system assumes ions have a desired known charge state. In addition, the uniformity of the ion beam can be disturbed by undesired charge exchanges. For instance, the ion beam may be initially set up in low vacuum conditions. During ion implantation in a high vacuum condition, the pressure may change due to gas evolving from the wafer and the beam uniformity may be disrupted by the redistribution of beamlets.
Two conventional solutions to lessen the amount of contamination ions striking the target wafer are to increase the dispersion angle of the scanner and the angle corrector magnet and/or to increase the drift length downstream of the angle corrector magnet by positioning the target wafer further from the angle corrector magnet. This would allow the contamination ions to sufficiently separate from the desired ions before reaching the target wafer. However, both of these solutions would require the ion implanter to increase its size considerably, which is typically not practical in a manufacturing facility where space is costly and/or unavailable.
Accordingly, there is a need to provide techniques for reducing contamination during ion implantation which overcomes the above-described inadequacies and shortcomings.